Ground/Power Planes

[Stellarcore]

Hey Folks,

I've been playing around with some PCB designs. I've used ground planes in the past (on top and bottom), and had no problems with it (except occasionally shorting things out with my clumsy meter probes).

I was thinking of trying on my next board a ground plane on the bottom side and a Vcc plane on the top side. Upon thinking about it, I dont think this should cause any problems, and in fact should be good; any added capacitance (however minor) would act like a big (as in geometry) filter cap.

Anybody have any thoughts regarding the use of ground or power planes on a PCB? What are some of the benefits and detriments? Do you think it makes routing easier? It sure makes the board look nicer imho. Let me know what you think.

 

[RadioRon]

The practice of using power and ground planes is common in the industry and is based on sound principles, so there is nothing wrong with you doing it. There are some pitfalls to consider and possibly avoid though.

A practical problem that I've faced many times is in putting the power plane on the top side, spread around components and traces. Often I find the need to connect things to ground while debugging a board. Things like probe connections, coax connections, shielding and shunt components all benefit from having a handy ground plane on the component side. So I've got into the habit of putting ground pours around my topside circuitry where convenient in addition to putting a ground plane on the bottom. I prefer to put my power plane on an internal layer, or distributing power via large traces rather than planes if only a two-layer board.

I was working on a board recently where the designer had flooded the component side with Vcc plane. It is annoying to work on this board for the reasons mentioned above. And if someone unfamiliar with the board were to work on it, they might mistake the Vcc plane for a ground plane and connect a scope probe to it and cause a short.

Another problem that I see time and time again when others do board layouts is the excessive degree to which they compromise the ground plane by cutting through it with signal and power traces. This is especially a problem if you have a high speed logic board with many high speed logic ICs, or if you are working at high frequencies on a radio circuit. If you are working with complex high speed logic, or are doing an RF board, then don't cut your ground plane up. Small round openings for vias and such are fine, they don't cause any problems, but long cuts through the plane will force high frequency current to go all the way around the cut, a relatively long distance.

 

[Gordz]

It is a good way to make your PCB, lower noise when building switching PSU's and using ADC's. But like RadioRon says, too many 'cuts' in this and it can create problems especialy at higher frequencies. Flooded Vcc planes are extremely annoying...

 

[ericgibbs]

hi,
This application note has a number of tips.